1. Field of the Invention
This invention relates to a hydraulic control system for an automatic transmission of a motor vehicle, more particularly to a hydraulic control system for an automatic change-speed transmission of a motor vehicle.
2. Description of Prior Art
In the automatic transmissions of motor vehicles, a number of gear ratios are established generally by coupling and uncoupling a number of friction elements. During the shift operation from one gear ratio to another, the power from the engine is still flowing through the transmission so that it is very important to effect the switching from one frictional element to another with a proper timing. This is because improperly timed switching of the frictional element causes mechanical shocks or impacts at the time of coupling, places the engine in overrun conditions, deteriorates the maneuverability of the vehicle, or gives rise to troubles in the transmission per se.
In order to avoid the mechanical shocks which occur when shifting the transmission from one gear ratio to another, there has thus far been employed a method of using a frictional coupling element in combination with a one-way clutch. This method, however, is feasible only on such large-sized transmissions as will require a disadvantageously high production cost.